Apraclonidine and clonidine are used to achieve what effect on intraocular pressure?

• A. Decrease IOP
• B. Increase IOP
• C. No change
• D. Fluctuate

Answer: (A)

Lowering intraocular pressure is achieved because these drugs are alpha-2 adrenergic agonists that act on the ciliary body to reduce aqueous humor production. Activation of alpha-2 receptors inhibits adenylate cyclase via Gi, lowering cAMP in the nonpigmented ciliary epithelium and thereby decreasing aqueous humor secretion. The net effect is a fall in intraocular pressure (they may have a minor effect on outflow, but the dominant action is reduced production). They do not reliably increase IOP, cause no change, or cause primary fluctuation, so decreasing IOP is the best answer.